Emmetropization in chick eyes: optimizing refractive state by visual feedback control

Abstract

From studies of refractive development in humans, it is difficult to derive mechanisms of emmetropization because refractive errors may both be pre-programmed genetically but may also be acquired from environmental factors. By contrast, animal models offer the opportunity to study the cause and effect relationship in controlled conditions. Animals with rapidly growing eyes and good acuity are most appropriate. Many intriguing results were recently obtained in the chicken: it was found that eyes can correct for refractive errors imposed by spectacle lenses by changes in axial eye growth, and this result can also be obtained without functional accommodation. Removing high spatial frequencies from the retinal image during development by translucent occluders induces axial refractive errors (deprivation myopia), and deprivation myopia can be restricted to parts of the visual field only if the deprivation was local. The occurrence of deprivation myopia does not require an intact optic nerve. Eyes even recover from induced myopia and hyperopia without intact optic nerve. The results suggest that (1) refractive state is guided by the quality of the retinal image, and (2) information from the retinal image is used locally to modulate growth of the underlying sclera.